Systems and Techniques for Suppressing Backward Lasing in High-Power Cascaded Raman Fiber Lasers

ABSTRACT

In a light amplification system and technique, a pump source provides pump power at a source wavelength. The pump power is launched as an input into a cascaded Raman resonator. A wavelength-dependent loss element is connected such that it precedes the cascaded Raman resonator. The wavelength-dependent loss element is configured to transmit light power at the source wavelength with low loss, and to provide high loss at the first Stokes shift. The wavelength-dependent loss element prevents buildup of light power between the pump source and the cascaded Raman resonator, thereby preventing backward propagation of light power back into the pump source.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the priority benefit of United States Provisional Patent Application Ser. No. 61/177,058, filed on May 11, 2009, which is owned by the assignee of the present application, and which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical fiber devices and methods, and in particular to improved systems and techniques for suppressing backward lasing in high-power cascaded Raman fiber lasers.

2. Background Art

Stimulated Raman scattering in optical fibers is a useful effect that can be used to provide nonlinear gain at wavelength regions where rare-earth doped fibers do not operate. A cladding-pumped, Yb-doped fiber can serve as a brightness converter to convert high-power multi-mode diodes at 915 nm or 975 nm to single-mode radiation in the region of 1.0 to 1.2 micrometers. This can then be used to pump a cascaded Raman resonator to shift the wavelength of the Yb laser output over a broad range, by using multiple Stokes shifts. In this manner, high-power single mode radiation can be generated at, for example, 1480 nm which can then be used to pump high power erbium-doped fiber amplifiers in the fundamental mode. This technique is described in J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. Headley, and D. DiGiovanni, “Picosecond Pulse Amplification in a Core-Pumped Large-Mode-Area Erbium Fiber,” Opt. Lett. 32, 2429-2431 (2007), which is incorporated herein by reference in its entirety.

FIG. 1 shows a diagram an exemplary 40 W 1480 nm system 20, in which a Yb-doped fiber laser is used to pump a cascaded Raman resonator. Multiple multi-mode 915 or 975 nm diode lasers are combined through a tapered fiber bundle (TFB) and launched into a double-clad, Yb-doped fiber. The double-clad, Yb-doped fiber guides the signal light in a single mode core, and the pump light in an inner cladding. Fiber Bragg gratings form high reflector (FIR) and output coupler (OC) in the Yb fiber laser resonator.

The output of the Yb fiber laser is launched into the Raman fiber resonator. The Raman fiber comprises a. small effective area fiber with normal dispersion. The normal dispersion prevents modulation instability that would lead to supercontinuum generation at high powers. The small effective area leads to high Raman gain, and consequently multiple higher-order Stokes shifts can be generated in the cascaded Raman resonator (CRR), where multiple resonators are made up of multiple fiber-Bragg gratings separated in wavelength by the Raman Stokes shift. An output coupler at the final desired Stokes shift couples the radiation out of the fiber and an additional pump reflector recycles unused Yb radiation for increased efficiency. Note that the wavelengths given in FIG. 1 are for illustration purposes only, and the exact wavelengths used will depend on the final desired wavelength.

The multiple reflectors at various wavelengths and positions in the schematic in FIG. 1 combine to create coupled cavities. For example, note that the Raman input grating (RIG) set has a high reflector at 1175 nm, which is the first Stokes shift of 1117 nm. While this reflector is intended to provide circulation of 1175 nm radiation inside the Raman fiber, it can also cause lasing at 1175 nm in the double-clad Yb-doped fiber if the 1117 nm power becomes high enough since 1175 nm radiation is within the bandwidth of both the Yb ionic gain and Raman gain from the 1117 nm radiation. This backward lasing 1175 nm can then destabilize the Yb-doped fiber laser. Ultimately, it can cause pulsing of the laser which can lead to component failure.

SUMMARY OF THE INVENTION

These and other issues of the prior art are addressed by the present invention, one aspect of which provides a light amplification system and technique, in which backward lasing is suppressed.

According to one practice of the invention, a pump source provides pump power at a source wavelength. The pump power is launched as an input into a cascaded Raman resonator. A wavelength-dependent loss element is connected such that it precedes the cascaded Raman resonator. The wavelength-dependent loss element is configured to transmit light power at the source wavelength with low loss, and to provide high loss at the first Stokes shift. The wavelength-dependent loss element prevents buildup of light power between the pump source and the cascaded Raman resonator, thereby preventing backward propagation of light power back into the pump source.

A further aspect of the invention is directed to systems and techniques for scaling to higher powers through the use of an amplifier fiber with a larger modefield diameter, and a wavelength-dependent loss element with a broader bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a cascaded Raman resonator according to the prior art.

FIG. 2 is a general diagram of a system according to a first aspect of the invention.

FIG. 3 is a diagram of an exemplary system incorporating the overall structure of the system shown in FIG. 2.

FIG. 4 is a more detailed diagram of the cascaded Raman resonator in the system shown in FIG. 3.

FIGS. 5A-5B show a diagram of a testing setup based on the MOPA configuration shown in FIGS. 3 and 4.

FIGS. 6A-6D are a series of graphs of measurements taken using the testing setup shown in FIGS. 5A-5B.

FIG. 7 is a graph illustrating the measured insertion loss of a long-period grating used in the testing setup shown in FIGS. 5A-5B.

FIGS. 8A-8B show a testing setup for testing backward Stokes lasing and pulsing at higher powers.

FIG. 9A is a graph illustrating backward propagating power as a function of 1480 nm output power, and FIG. 9B is a graph showing oscillator time traces for different output powers for the testing setup shown in FIGS. 8A-8B.

FIGS. 9C and 9D show the output power of the cladding-pumped fiber laser and cascaded Raman resonator of the testing setup shown in FIGS. 8A-8B.

FIG. 10 is a graph comparing LPG insertion loss with the spectrum of the backward propagating Stokes wavelength at a maximum Raman output power of 78W.

FIG. 11 is a flowchart of a general technique according to various described aspects of the invention.

DETAILED DESCRIPTION

An aspect of the invention provides systems and techniques for suppressing backward lasing in high-power cascaded Raman fiber lasers. As described herein, suppression of backward lasing is accomplished by identifying signatures that point to the onset of backward lasing. The identification of these signatures is a very powerful technique. The temporal disturbances caused by backward lasing can lead to pulsing, which can destroy components at higher powers.

A further aspect of the invention provides a Raman lasing system in which a wavelength-dependent loss element is used to eliminate backward lasing from a cascaded Raman resonator by frustrating the buildup of radiation at the first Stokes shift. When a system according to the prior art, such as the FIG. 1 system discussed above, is operated at higher powers, this radiation building and backward lasing may result in failure, for example, of the pump laser high reflector (HR) when external devices are connected. According to an aspect of the invention, a suitable fiber-based loss element connected between the Yb system and the Raman laser is used to significantly improve system reliability.

According to a further aspect of the invention, a large modefield diameter (MFD) fiber is used in a Raman lasing system to increase the Raman threshold. The large MFD fiber is combined with wavelength-selective filtering in order to reduce potential sources of feedback.

It should be noted that the above-described issues with respect to backwards lasing may be addressed in other ways. In one alternative approach, a depressed-clad W-shaped index profile is used in conjunction with a Yb amplifier fiber to achieve a fundamental mode cutoff that gives rise to high loss at the first Stokes shift and low loss at the amplifier output. This approach is described in United States Provisional Patent Application Ser. No. 61/177,058, filed on May 11, 2009, which is owned by the assignee of the present application, and which is incorporated herein by reference in its entirety.

FIG. 2 shows a general diagram of a system 100 according to the invention, which includes a Raman pump source 120 that launches pump power 121 through a wavelength-selective loss element 140 into a cascaded Raman resonator (CRR) 160, producing a system output 180 at a desired wavelength.

Pump source 120 may be implemented using a number of different structures, including a single oscillator configuration, such as that shown in FIG. 1, and a master oscillator power amplifier (MOPA) configuration, discussed below. Pump source 120 provides pump power 121 at a specified wavelength, which in the present example is 1117 nm.

Pump power 121 travels through loss element 140 before it is launched as a pump power input into CRR 160. As discussed in greater detail below, wavelength-sensitive loss element 140 has low loss at the pump wavelength, and high loss at the first Stokes shift.

CRR 160 comprises a length of Raman-active fiber 162, a Raman input grating set RIG1, and a Raman output grating set ROG1 that together form a nested series of laser cavities 164. As light propagates through the nested laser cavities 164, it is subjected to a series of Stokes shifts in order to produce a system output 180 having a desired wavelength. It is well known to those skilled in the art that Raman resonators may be constructed using alternative architectures and wavelength selective elements, such as use of fused-fiber WDM couplers or thin-film filters to construct WDM loop mirrors. It will also be appreciated that the CRR could be configured in a linear cavity or as a unidirectional ring cavity or as a bidirectional ring cavity. It will also be appreciated that the CRR can be configured to operate as a laser, or, by leaving off the final set of reflectors and instead by injecting a signal into the CRR at the final wavelength, the CRR can be configured to operate as an amplifier. The present discussion focuses on linear resonators constructed using Bragg grating reflectors for illustration purposes only, and the basic feature of suppressing backward propagating light from the CRR 160 to pump source 120 is unchanged.

Wavelength-selective loss element 140 substantially eliminated backward lasing at the first Stokes shift by frustrating the buildup of radiation at the first Stokes shift using wavelength-sensitive loss element 140, which has been configured to have high loss for the first Stokes order light while maintaining low loss for the pump input light. In this manner, the Raman input grating set RIG1 becomes invisible to the Yb-doped laser system while providing high reflectivity for the Raman laser.

In the present example, loss element 140 is provided by a long-period grating LPG1. A long-period grating is a wavelength-dependent device that couples light, at certain wavelengths, from a guided mode into higher-order cladding modes, where the light is lost due to absorption and scattering.

In this example, LPG1 is configured to transmit light at the pump wavelength, i.e., 1117 nm, with little or no loss. Grating LPG1 is further configured to provide high loss at the first Stokes shift, i.e., 1175 nm.

Once the 1117 nm pump power input is launched as an input into the CRR 160, it undergoes one or more Stokes shifts, the first of which is at 1175 nm. It will be seen from FIG. 2 that all light propagating between the pump source 120 and the CRR 160 passes through loss element 140. Because loss element 140 is configured to provide high loss at the first Stokes shift, i.e., 1175 nm, it will be seen that there will be a significant reduction of radiation buildup at 1175 nm, and thus a significant reduction of backward-propagating radiation into the pump power source 120.

While a long-period grating is illustrated in the FIG. 2 system, other wavelength-dependent devices could also be used, such as tilted fiber Bragg gratings, fused fiber wavelength division multiplexers (WDMs), a length of fundamental mode cutoff fiber, or the like. Any such device must be capable of handling high powers. In principle an optical isolator would work in this application as well, but in practice, with present technology, fiber coupled optical isolators do not exist that can handle the high pump powers (>100 W) required to reach high power output from the CRR.

It has been found that a narrow-bandwidth LPG having a center wavelength matched to the wavelength of the reflector grating in the RIG works very well at increasing the threshold of backward Stokes lasing. Broadband operation for the wavelength-dependent loss element 140 is not necessary, because the undesired feedback comes from CRR 160, which has a known, specific wavelength response, i.e., the Stokes shift.

FIG. 3 shows a diagram of an exemplary system 200 incorporating the overall structure of the system 100 shown in FIG. 2. System 200 includes a pump power source 220, a wavelength-sensitive loss element 240, and a cascaded Raman resonator 260. System 200 provides a high power output 280 at a desired wavelength, which in this case is 1480 nm.

Pump power source 220 is implemented using a master oscillator power amplifier (MOPA) configuration, in which amplifier components are optically isolated from an oscillator laser. Such a configuration is described in U.S. Provisional Patent Application Ser. No. 61/177,058, filed on May 11, 2009, which is owned by the assignee of the present application, and which is incorporated herein by reference in its entirety.

In system 200, pump source 220 comprises a master oscillator 221 and a power amplifier 230 that are optically connected together by a suitable coupler 225, such as a wavelength division multiplexer, or like device, that isolates the oscillator 221 from backward propagating radiation from the amplifier 230 or the CRR 260. This isolation allows the master oscillator 221 to be operated at low power and the amplifier 230 to be operated at high power, thereby protecting the components of the master oscillator 221 from damage.

The pump power from pump source 220 is then launched into cascaded Raman resonator 260 through wavelength-dependent loss element 240. CRR 260 comprises a nested series of Raman cavities 264 formed by a Raman input grating set RIG2, a length of Raman active fiber 262, and Raman output grating set ROG2.

FIG. 4 shows a more detailed diagram of the cascaded Raman resonator 260. As shown in FIG. 3, Raman input grating set RIG2 comprises high reflectors HR21-HR25, and ROG2 comprises high reflectors HR26-HR30 and output coupler OC21. Gratings HR21-HR25 in RIG2 and corresponding gratings H27-HR30 and OC21 in ROG2 form a nested. series of wavelength-matched grating pairs that create a nested series of laser cavities with wavelengths corresponding to the Stokes shifts, creating a stepwise transition from the pump power input wavelength to the desired output wavelength.

FIGS. 5A-5B show a diagram of a testing setup 300 based on the MOPA configuration shown in FIGS. 3 and 4, that was used to measure the impact of adding the RIG to a high-power Yb fiber laser system. Testing setup 300 comprises a master oscillator 320 (FIG. 5A) and a power amplifier 340 (FIG. 5B). A long-period grating LPG3 is connected to the output of amplifier 340.

In order to characterize the performance of the testing setup, three sets of power meters and optical spectrum analyzers PM31/OSA31, PM32/OSA32, PM33/OSA33 are connected into testing setup 300. First set PM31/OSA31 is connected to the system output. Second and third sets PM32/OSA32 and PM33/OSA33 are connected to 1117/1480 tap WDM 330 to measure, respectively, forward and backward propagation between the oscillator 320 and the amplifier 340.

As shown in FIG. 5A, PM32/OSA32 are connected to tap WDM 330 using a coupler C32 that directs 10% of incoming light to OSA32 and 90% to PM32. PM33/OSA33 are connected to tap WDM 330 using a coupler C33 that directs 1% of incoming light to OSA33 and 99% to PM33.

In order to analyze the impact of RIG3 on system 300, measurements were first taken without RIG3 connected into the system. Further measurements were taken with RIG3 connected between LPG3 and PM31/OSA31. FIGS. 6A-6D are a series of graphs 410-440 of these measurements.

FIG. 6A is a graph 410 of power measurements (mW) taken at the third power meter PM33 at increasing levels of amplifier current (A). Trace 411 shows measurements taken without RIG3 connected into the system, and trace 412 shows measurements taken with RIG3 connected into the system.

FIG. 6B is a graph 420 of power measurements (mW) taken at the first power meter PM31 at increasing levels of amplifier current (A). Trace 421 shows measurements taken without RIG3 connected into the system, and trace 422 shows measurements taken with RIG3 connected into the system.

FIGS. 6C and 6D are graphs 430 and 440 of spectra generated, respectively, at the third and second optical spectrum analyzers OSA33 and OSA32, showing the relationship between power (dB) and wavelength (nm), with RIG3 connected into the system. Spectra 431 and 441 were generated at an amplifier drive current of 0A, spectra 432 and 442 were generated at 10A; spectra 433 and 443 were generated at 20A; and spectra 434 and 444 were generated at 30A.

As shown by trace 411 in FIG. 6A and trace 421 in 6B, without RIG3 in place the output power of the Yb amplifier is only limited by available pump current. Very little backward propagating power is observable, and turning up the amplifier current has no measurable impact on the oscillator spectrum (not shown). Thus, without RIG3, there is a generally uninterrupted increase in power output for amplifier currents ranging from OA to approximately 45A.

The situation changes substantially when RIG3 is added to the system output, however. As shown by trace 412 in FIG. 6A, at 25-30A of pump current, the backward propagating power spikes upwards, reaching approximately 300 mW at 30A. As shown by trace 422 in FIG. 6B, there is a substantial drop-off in output power when the amplifier current reaches 30A.

From spectra 431-434 in FIG. 6C, the spike in power shown in FIG. 6A can be seen to correspond to a large increase in the 1175 nm backward-propagating component. Cascade lasing to the second Stokes order could also be observed in the forward propagating oscillator spectra 441-444 shown in FIG. 6D. Other effects observed included a large temperature increase in the amplifier tapered fiber bundle of greater than 30° C.

From these results, it can be concluded that even though the RIG does not directly reflect the 1117 nm light from the Yb laser, backward lasing of light at the 1175 nm first Stokes shift can occur at high pump powers. This backward propagating 1175 light essentially limits the amount of output power this is achievable from the amplifier while still maintaining stable operation of the Yb oscillator.

Thus, as described above with respect to FIGS. 2 and 3, to overcome this limitation a wavelength-dependent loss element, e.g., filter 140 in FIG. 2 and filter 240 in FIG. 3 is connected into the system such that it precedes the cascaded Raman resonator 260. According to an aspect of the invention, the wavelength-dependent loss element comprises a discrete element between the pump source and the cascaded Raman resonator. According to a further aspect of the invention, the wavelength-dependent loss element comprises other types of structures. For example, in one practice of the invention, the wavelength-dependent loss element comprises a filter fiber with high loss at the first Stokes shift.

In FIGS. 2 and 3, a long-period grating (LPG) is shown, although other wavelength-dependent filters, such as tilted fiber Bragg gratings, fused fiber WDMs, appropriately doped attenuating fibers, or the like, may also be used. Theoretically, an optical isolator could also be used. However, current commercial isolators cannot handle the required power levels and may introduce unacceptable loss at the Yb laser wavelength.

As mentioned above, another possibility for filtering would be to use a depressed-clad index profile in the Yb amplifier fiber 232, or the fiber connecting the pump source 220 to Raman resonator 260, for fundamental mode cutoff at long wavelengths. The Yb amplifier fiber 232, for example, would then have high loss at 1175 nm and low loss at 1117 nm. The key component of the loss filter is that it has high loss at the same wavelength of the first Stokes wavelength used in the RIG, and that it have low loss at the Yb laser wavelength.

In the depicted systems, the long-period gratings (LPGs) were manufactured using the electric arc of a fusion splicer, although other techniques may also be used. The LPGs were designed to provide coupling between two different modes of a fiber by phase matching.

FIG. 7 is a graph 450 illustrating the measured insertion loss of the LPG. As shown by trace 451, it will be seen that there is approximately 20 dB of loss at 1175 nm and less than 0.1 dB at 1117 nm. It has been observed that, with the LPG in place, the amplifier may be turned up to full power with no change in the oscillator spectrum observed, compared to the spectrum measured with the amplifier off. Furthermore, no spike in backward-propagating power was observed once the LPG was inserted into the system. This experiment confirms the importance of isolating the RIG reflectors from the Yb fiber laser system in order to enhance system stability.

A further aspect of the invention is directed to additional systems and techniques for scaling to even higher output powers. It has been found that the narrow-bandwidth LPG matched to the reflector in the RIG works very well at increasing the threshold of backward Stokes lasing. However, as the output power of the Raman laser increases beyond a certain level, the narrow-bandwidth LPG is no longer sufficient and backward Stokes lasing again is observed.

FIGS. 8A-8B show a testing setup 500 for testing backward Stokes lasing and pulsing at higher powers. Setup 500 includes a cladding-pumped fiber laser (CPFL) 520 and amplifier 530 that provide pump power to a cascaded Raman resonator 560. A wavelength-selective loss element 540 is connected between amplifier 530 and CRR 560.

Measurements were taken using: (1) optical spectrum analyzer OSA51 and power meter PM51 connected to the output of CRR 560; (2) optical spectrum analyzer OSA52 and fast photodiode and oscilloscope 531, coupled to tap WDM 525 to measure forward propagating radiation; (3) optical spectrum analyzer OSA53 and power meter PM52, coupled to tap WDM 525 to measure backward propagating radiation; and (4) power meter PM53 connected to the input of CPFL 520.

For an initial set of measurements, both the oscillator and the amplifier were constructed using a Yb-doped fiber with a 6 μm mode-field diameter (MFD).

FIG. 9A is a graph 610 illustrating backward propagating power as a function of 1480 nm output power. Trace 611 shows backward propagating power at PM52, i.e., through tap WDM 525; trace 612 shows backward propagating power at PM 53, i.e., through master oscillator 520; and trace 613 shows total backward propagating power.

FIG. 9B is a graph 620 showing oscillator time traces, taken at fast photodiode and oscilloscope 531, for different CPFL output powers. Time trace 621 was taken at 44W; time trace 622 was taken at 55W; and time trace 623 was taken at 58W.

As shown in FIGS. 9A and 9B, at around 58 W of 1480 nm output power, two distinct signatures of backward Stokes lasing can be seen. First, as shown in FIG. 9A, the backward propagating power begins to rapidly increase. Second, as shown in FIG. 9B, the Yb oscillator begins to show pulsing behavior in the time trace. So, while the backward lasing is suppressed up to relatively high powers, at a certain point, a narrow-bandwidth LPG is no longer sufficient.

Thus, a further aspect of the invention is directed to design modifications that allow scaling to higher powers. Because there is a certain amount of ionic gain from Yb at 1175 nm, it is not immediately obvious that scaling the mode-field diameter of the Yb laser will allow for increasing the threshold for backwards lasing. In fact, however, there is a combination of Raman gain and ionic gain at 1175 in the Yb power amplifier. Therefore a power amplifier was implemented using a Yb-doped, double-clad fiber with an increased modefield diameter (MFD) of 11 μm. Although this MFD is relatively large for a Yb-doped fiber, it still supports single-mode operation. Thus, the backward lasing threshold is maximized while maintaining fundamental mode propagation.

FIGS. 9C and 9D show the output power of the cladding-pumped fiber laser (CPFL) and cascaded Raman resonator (CRR), where the MOPA pump source was constructed using 26 m of Yb amplifier fiber with a MFD of 11 μm. The 1480 nm output power was increased to 73 W before backward Stokes lasing was observed. This result compares favorably with a system using a 6 μm MFD diameter amplifier fiber where 58W at 1480 nm was achieved before backwards lasing was observed. Note that in FIGS. 9C and 9D, the plot of backward propagating power as a function of output power also shows a rapid increase in backward propagating power at around 70-75 W output power. At this power, the oscillator time trace (not shown) also showed indications of temporal pulsing.

Scaling to even higher powers would require further increase of the backward Stokes lasing threshold. Significant improvement can be obtained from a better LPG filter. Lasing lines in the Raman cavity are significantly broadened by nonlinear processes, and in fact the output radiation from intermediate Stokes orders are much broader than the FBG high reflectors.

FIG. 10 is a graph 650 comparing LPG insertion loss (trace 651) with the spectrum (trace 652) of the backward propagating Stokes wavelength at a maximum Raman output power of 78W. The 10 dB bandwidth of the LPG is only 2 nm, whereas the 10 dB bandwidth of the 1175 nm peak is greater than 10 nm. In fact, the backward propagating radiation peaks at 1176 nm, away from the LPG loss peak. Therefore, while a narrow filter is effective at suppressing reflections from the RIG 1175 nm HR, a broader filter is required to suppress radiation from the Raman cavity that leaks around the HR.

FIG. 11 is a flowchart of an overall technique 700 according to various aspects of the invention described herein.

Box 701: Use a pump source to provide pump power at a source wavelength.

Box 702: Launch the pump power into a cascaded Raman resonator into which the pump power is launched as an input, wherein the cascaded Raman resonator comprises sets of input and output gratings defining a nested series of Raman cavities producing a first Stokes shift in the pump power input followed by a series of higher order Stokes shifts, thereby providing a stepwise transition from the source wavelength to an output wavelength.

Box 703: Connect a wavelength-dependent loss element between the pump source and the cascaded Raman resonator.

Box 704: Configure the wavelength-dependent loss element to transmit light power at the source wavelength with low loss, and to provide high loss at the first Stokes shift, whereby the wavelength-dependent loss element prevents buildup of light power between the pump source and the cascaded Raman resonator, thereby preventing backward propagation of light power back into the pump source.

It is noted that the Raman gain bandwidth is quite large and that the reflectors can be positioned anywhere within the gain bandwidth, not necessarily at the peak of the gain.

The above described systems and techniques are applicable in a number of other contexts including, but not limited to: both linear and ring Raman resonators; a Raman amplifier architecture; a double-pump system including a second pump that is non-resonant with any of the Raman cavities, but that is still within the Raman gain bandwidth; hitting a frequency-doubling crystal, for which a polarized output with a narrow linewidth is beneficial; pulsed or modulated operation, as used for example in a parametric system; and the like.

With respect to Raman amplifiers, it is noted that their architectures are typically similar to those of Raman lasers, except that the amplifier Raman cavity is constructed without the last Stokes shift and output coupler. Also, a seed laser is coupled into the Raman cavity at the last Stokes shift. The seed input from the seed source can be injected into the amplifier at different locations. The seed laser controls a number of amplifier properties, such as polarized output, narrow linewidth, tunability, and the like.

While the foregoing description includes details which will enable those skilled in the art to practice the invention, it should be recognized that the description is illustrative in nature and that many modifications and variations thereof will be apparent to those skilled in the art having the benefit of these teachings. It is accordingly intended that the invention herein be defined solely by the claims appended hereto and that the claims be interpreted as broadly as permitted by the prior art. 

1. A light amplification system, comprising: a pump source, providing pump power at a source wavelength; and a cascaded Raman resonator into which the pump power is launched as an input, wherein the cascaded Raman resonator comprises one or more nested Raman cavities for creating a first Stokes shift in the pump power, thereby providing a stepwise transition from the source wavelength to an output wavelength; and a wavelength-dependent loss element preceding the cascaded Raman resonator, wherein the wavelength-dependent loss element is configured to transmit light power at the source wavelength with low loss, and to provide high loss at a wavelength approximately equal to the wavelength of the first Stokes shift reflector in the Raman resonator, whereby the wavelength-dependent loss element reduces backward propagation of light power back in the pump source.
 2. The light amplification system of claim 1, wherein the nested Raman cavities are provided by a Raman input grating set and a Raman output grating set.
 3. The light amplification system of claim 1, wherein the nested Raman cavities are provided by a WDM loop mirror.
 4. The light amplification system of claim 1, wherein the wavelength-dependent loss element comprises a discrete component between the pump source and the cascaded Raman resonator.
 5. The light amplification system of claim 4, wherein the wavelength-dependent loss element comprises a long-period grating.
 6. The light amplification system of claim 5, wherein the long-period grating has a narrow bandwidth and a center wavelength matched to a wavelength of a first input grating in the cascaded Raman resonator.
 7. The light amplification system of claim 5, wherein the wavelength-selective loss element comprises a long-period grating having a bandwidth that sufficiently broad to suppress radiation at higher-order Stokes shifts, while maintaining low loss at the pump wavelength.
 8. The light amplification system of claim 7, wherein the 10 dB bandwidth of the long-period grating is greater than 10 nm.
 9. The light amplification system of claim 4, wherein the wavelength-dependent loss element comprises a tilted fiber Bragg grating.
 10. The light amplification system of claim 4, wherein the wavelength-dependent loss element comprises a fused-fiber wavelength division multiplexer.
 11. The light amplification system of claim 2, wherein the wavelength-dependent loss element comprises a fundamental mode cutoff fiber.
 12. The light amplification system of claim 1, wherein the wavelength-dependent loss element comprises a filter fiber with high loss at the first Stokes shift.
 13. The light amplification system of claim 1, wherein the pump source is configured as a master oscillator power amplifier.
 14. The light amplification system of claim 13, wherein the power amplifier comprises a double-clad fiber having a modefield diameter that maximizes a backward lasing threshold while supporting single-mode operation.
 15. The light amplification system of claim 14, wherein the power amplifier comprises a Yb-doped double-clad fiber having a modefield diameter of 11 μm.
 16. The light amplification system of claim 13, wherein the power amplifier comprises a double-clad fiber having a modefield diameter that maximizes a backward lasing threshold while supporting fundamental mode and higher-order mode propagation.
 17. A light amplification method, comprising: (a) using a pump source to provide pump power at a source wavelength; (b) launching the pumping light power into a cascaded Raman resonator, wherein the cascaded Raman resonator comprises one or more Raman cavities for creating a first Stokes shift in the pump power input followed by a series of higher order Stokes shifts, thereby providing a stepwise transition from the source wavelength to an output wavelength; (c) connecting a wavelength-dependent loss element such that it precedes the cascaded Raman resonator; and (d) configuring the wavelength-dependent loss element to transmit light power at the source wavelength with low loss, and to provide high loss at the first Stokes shift, whereby the wavelength-dependent loss element prevents buildup of light power between the pump source and the cascaded Raman resonator, thereby preventing backward propagation of light power back into the pump source.
 18. The method of claim 17, wherein the nested Raman cavities are provided by a Raman input grating set and a Raman output grating.
 19. The method of claim 17, wherein the nested Raman cavities are provided by a WDM loop mirror.
 20. The method of claim 17, wherein the wavelength-dependent loss element comprises a discrete component between the pump source and the cascaded Raman resonator.
 21. The method of claim 20, wherein the wavelength-dependent loss element comprises a long-period grating.
 22. The method of claim 21, wherein the long-period grating has a narrow bandwidth matched to a first input grating in the cascaded Raman resonator.
 23. The method of claim 21, wherein the wavelength-selective loss element comprises a long-period grating having a bandwidth that sufficiently broad to suppress radiation at higher-order Stokes shifts, while maintaining low loss at the pump wavelength.
 24. The method of claim 23, wherein the 10 dB bandwidth of the long-period grating is greater than 10 nm.
 25. The method of claim 20, wherein the wavelength-dependent loss element comprises a tilted fiber Bragg grating.
 26. The method of claim 20, wherein the wavelength-dependent loss element comprises a fused-fiber wavelength division multiplexer.
 27. The method of claim 17, wherein the wavelength-dependent loss element comprises a fundamental mode cutoff fiber.
 28. The method of claim 17, wherein the wavelength-dependent loss element comprises a filter fiber with high loss at the first Stokes shift.
 29. The method of claim 17, wherein the pump source is configured as a master oscillator power amplifier.
 30. The method of claim 29, wherein the power amplifier comprises a double-clad fiber having a modefield diameter that maximizes a backward lasing threshold while supporting single-mode operation.
 31. The method of claim 30, wherein the power amplifier comprises a Yb-doped double-clad fiber having a modefield diameter of 11 μm.
 32. The method of claim 29, wherein the power amplifier comprises a double-clad fiber having a modefield diameter that maximizes a backward lasing threshold while supporting fundamental mode and higher-order mode propagation. 